The objectives of this proposal are in four major areas of study: I. To determine the properties and biological behavior of two DNA polymerases in differentiating skeletal muscle cells. One of the polymerases has properties not previously described for DNA polymerase in eukaryotic cells. Both enzymes will be purified and characterized, and their presence will be correlated with proliferative and non- proliferative events during skeletal muscle differentiation. II. To determine the relationship of "cytoplasmic DNA" synthesis to events that occur during the differentiation of skeletal muscle cells. III. To determine the relationship of cell proliferation to the transition from cell determination to overt cell differentiation. IV. To determine the mechanism by which creatine regulates the synthesis of myosin and actin.